The present invention relates to oriented rubber-reinforced polystyrene film that has a preferential orientation in the stretched direction and a shrink film comprising such a polystyrene film, preferably multilayer film which is preferably coextruded.
Shrink labels generally fall into two categories: roll-on shrink-on labels and sleeve-type labels; sleeve labels are also sometimes referred to as tube labels. Roll-on shrink-on labels are film sheets that wrap around a container. Sleeve labels are tubular in configuration and fit around a container by placement over the container, such that the container is surrounded by the tube. Application of heat to a shrink label that is around a container causes the label to shrink and conform to the container.
To conform to a container, each type of label must shrink preferentially (that is, to a greater extent than in any other direction) in the direction extending circumferentially around the container. Roll-on shrink-on films usually reside on a container with the machine direction (MD) of the film extending circumferentially around the container. Hence, roll-on shrink-on films primarily shrink in the film's machine direction (MD) due to preferential machine direction orientation (MDO). In contrast, sleeve labels usually reside on a container with the label's transverse direction (TD) extending circumferentially around the container. Hence, sleeve labels shrink primarily in the film's transverse direction (TD) due to preferential transverse direction orientation (TDO).
Polystyrene (PS) is a particularly desirable polymer for shrink labels. Shrink label films of polypropylene (PP), for example, usually shrink only up to about 20 percent in any direction at a temperature below 120° C. The crystalline nature of PP requires heating above the PP's crystalline melt temperature to release additional orientation. In contrast, PS-based shrink label films only need to exceed the polymer's glass transition temperature (which generally is lower than PP's crystalline melt temperature) due to its amorphous character. Therefore, PS films can desirably provide greater shrink at lower processing temperatures than PP films.
Labels with glued seams require care to avoid shifting or loss of the label. Excess stress on the glued seam can cause the label to shift on the container or even separate from the container. Shrink labels, which either have no glue joint or have a glue joint that is extensively cured prior to application to a container, can tolerate greater stress. A very strong joint or seam can be formed in a polystyrene based film by using a solvent. These seams can tolerate greater stress during shrinkage. A seam in a PP shrink label cannot be formed using a solvent. Instead, glue is used. As a result the seams in PP labels are weaker and more likely to shift or be lost.
Additionally, PS retains a higher surface energy after corona treatment (often needed to render the surface of a polymer film suitable for printing) for extended periods of time relative to PP. Therefore, unlike PP films, corona treatment of PS films can occur during manufacture rather than just prior to printing into labels. Such flexibility in timing of corona treatment leads to significant efficiencies.
In contrast to copolyester and polyvinyl chloride (PVC) films, use of PS films facilitate bottle and label recyclability, as the lower density allows the label to be easily separated from the higher density (for example, polyester) bottles. Furthermore, the lower PS density advantageously provides a higher film yield, or more area/lb (or more area/kg) of film. Higher density label stocks, such as copolyester or PVC films, do not provide similar advantages.
Polystyrene-based shrink label films can include a high impact polystyrene (HIPS) component in order to improve label toughness (for example, tear resistance). However, rubber particles in a typical HIPS range have an average particle size of greater than one micrometer (see, for example, U.S. Pat. No. 6,897,260, column 4, lines 26-27). Large rubber particles tend to decrease clarity of a label film, interfering with the use of the film for reverse side printing (printing on the side of a label film proximate to the container so that it is readable through the film) as well as with viewing of the container or product through the label. Typical HIPS also contains greater than 7 percent rubber based on total HIPS weight. High concentrations of rubber can hinder the printability of a film, decrease clarity of a film, reduce dimensional stability and undesirably increase gel amount in a final film. However, in some situations such as small diameter bottles or bottle necks, HIPS alone may not supply sufficient toughness to avoid a tendency to split under stress.
However, styrene block copolymers that provide toughness often interfere with printing, seam sealing or both. Styrene block copolymers often contain higher portions of more expensive diene elastomers and are, therefore more expensive than high impact polystyrene (HIPS). Furthermore, relatively higher styrene content enhances recycle because styrene polymers separate from polymers typically used in making such containers as bottles by differences in density.
It would be desirable to have an oriented PS film that is suitable for shrink label applications, particularly having toughness sufficient for shrink labels as well as the ability to receive print and sealing by a variety of machinery. It would be desirable if such a film could serve as a shrink label that demonstrates circumferential shrink around a container comparable to that achieved with PVC or polyester. It would be desirable for the film to have seams more resistant to separation or shifting than glued seams in polypropylene film. It would be desirable to facilitate recycle of scrap, and edge trimmings. Further, it would be desirable to minimize total diene.